The present invention, in some embodiments thereof, relates to isolated polypeptides and polynucleotides and more particularly, but not exclusively, to methods of using same for increasing tolerance of a plant to abiotic stress, growth, biomass, vigor and/or yield of a plant.
Abiotic stress (ABS; also referred to as “environmental stress”) conditions such as salinity, drought, flood, suboptimal temperature and toxic chemical pollution, cause substantial damage to agricultural plants. Most plants have evolved strategies to protect themselves against these conditions. However, if the severity and duration of the stress conditions are too great, the effects on plant development, growth and yield are profound. Furthermore, most of the crop plants are highly susceptible to ABS and thus necessitate optimal growth conditions for commercial crop yields. Continuous exposure to stress causes major alterations in plant's metabolism which ultimately leads to cell death and consequently yield loss. Thus, despite extensive research and intensive crop-protection measures, losses due to abiotic stress conditions remain in the billions of dollars annually.
Drought is a gradual phenomenon, which involves periods of abnormally dry weather that persists long enough to produce serious hydrologic imbalances such as crop damage and water supply shortage. In severe cases, drought can last many years and result in devastating effects on agriculture and water supplies. With burgeoning population and chronic shortage of available fresh water, drought is not only the number one weather-related problem in agriculture, but it also ranks as one of the major natural disasters of all time, causing not only economic damage (e.g., losses from the US drought of 1988 exceeded $40 billion), but also loss of human lives, as in the 1984-1985 drought in the Horn of Africa which led to a famine that killed 750,000 people. Furthermore, drought is associated with increase susceptibility to various diseases.
For most crop plants, the land regions of the world are too arid. In addition, overuse of available water results in increased loss of agriculturally-usable land (desertification), and increase of salt accumulation in soils adds to the loss of available water in soils.
Salinity, high salt levels, affects one in five hectares of irrigated land. This condition is only expected to worsen, further reducing the availability of arable land and crop production, since none of the top five food crops, i.e., wheat, corn, rice, potatoes, and soybean, can tolerate excessive salt. Detrimental effects of salt on plants result from both water deficit which leads to osmotic stress (similar to drought stress) and the effect of excess sodium ions on critical biochemical processes. As with freezing and drought, high salt causes water deficit; and the presence of high salt makes it difficult for plant roots to extract water from their environment. Soil salinity is thus one of the more important variables that determine whether a plant may thrive. In many parts of the world, sizable land areas are uncultivable due to naturally high soil salinity. Thus, salination of soils that are used for agricultural production is a significant and increasing problem in regions that rely heavily on agriculture, and is worsen by over-utilization, over-fertilization and water shortage, typically caused by climatic change and the demands of increasing population. Salt tolerance is of particular importance early in a plant's lifecycle, since evaporation from the soil surface causes upward water movement, and salt accumulates in the upper soil layer where the seeds are placed. On the other hand, germination normally takes place at a salt concentration which is higher than the mean salt level in the whole soil profile.
Germination of many crops is sensitive to temperature. A gene that would enhance germination in hot conditions would be useful for crops that are planted late in the season or in hot climates. In addition, seedlings and mature plants that are exposed to excess heat may experience heat shock, which may arise in various organs, including leaves and particularly fruit, when transpiration is insufficient to overcome heat stress. Heat also damages cellular structures, including organelles and cytoskeleton, and impairs membrane function. Heat shock may produce a decrease in overall protein synthesis, accompanied by expression of heat shock proteins, e.g., chaperones, which are involved in refolding proteins denatured by heat.
Heat stress often accompanies conditions of low water availability. Heat itself is seen as an interacting stress and adds to the detrimental effects caused by water deficit conditions. Water Evaporative demand exhibits near exponential increases with increases in daytime temperatures and can result in high transpiration rates and low plant water potentials. High-temperature damage to pollen almost always occurs in conjunction with drought stress, and rarely occurs under well-watered conditions. Combined stress can alter plant metabolism in novel ways; therefore understanding the interaction between different stresses may be important for the development of strategies to enhance stress tolerance by genetic manipulation.
Excessive chilling conditions, e.g., low, but above freezing, temperatures affect crops of tropical origins, such as soybean, rice, maize, and cotton. Typical chilling damage includes wilting, necrosis, chlorosis or leakage of ions from cell membranes. The underlying mechanisms of chilling sensitivity are not completely understood yet, but probably involve the level of membrane saturation and other physiological deficiencies. For example, photoinhibition of photosynthesis (disruption of photosynthesis due to high light intensities) often occurs under clear atmospheric conditions subsequent to cold late summer/autumn nights. In addition, chilling may lead to yield losses and lower product quality through the delayed ripening of maize.
Water deficit is a common component of many plant stresses. Water deficit occurs in plant cells when the whole plant transpiration rate exceeds the water uptake. In addition to drought, other stresses, such as salinity and low temperature, produce cellular dehydration.
Salt and drought stress signal transduction consist of ionic and osmotic homeostasis signaling pathways. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. The osmotic component of salt stress involves complex plant reactions that overlap with drought and/or cold stress responses.
Common aspects of drought, cold and salt stress response [Reviewed in Xiong and Zhu (2002) Plant Cell Environ. 25: 131-139] include: (a) transient changes in the cytoplasmic calcium levels early in the signaling event [Knight, (2000) Int. Rev. Cytol. 195: 269-324; Sanders et al. (1999) Plant Cell 11: 691-706]; (b) signal transduction via mitogen-activated and/or calcium dependent protein kinases (CDPKs) and protein phosphatases [Merlot et al. (2001) Plant J. 25: 295-303; Tahtiharju and Palva (2001) Plant J. 26: 461-470]; (c) increases in abscisic acid levels in response to stress triggering a subset of responses; (d) inositol phosphates as signal molecules (at least for a subset of the stress responsive transcriptional changes [Xiong et al. (2001) Genes Dev. 15: 1971-1984]; (e) activation of phospholipases which in turn generates a diverse array of second messenger molecules, some of which might regulate the activity of stress responsive kinases [e.g., phospholipase D; Frank et al. (2000) Plant Cell 12: 111-124]; (f) induction of late embryogenesis abundant (LEA) type genes including the CRT/DRE responsive COR/RD genes; (g) increased levels of antioxidants and compatible osmolytes such as proline and soluble sugars [Hasegawa et al. (2000) Annu. Rev. Plant Mol. Plant Physiol. 51: 463-499)]; and (h) accumulation of reactive oxygen species such as superoxide, hydrogen peroxide, and hydroxyl radicals.
Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.
Several genes which increase tolerance to cold or salt stress can also improve drought stress protection, these include for example, the transcription factor AtCBF/DREB1, OsCDPK7 (Saijo et al. 2000, Plant J. 23: 319-327) or AVP1 (a vacuolar pyrophosphatase-proton pump, Gaxiola et al. 2001, Proc. Natl. Acad. Sci. USA 98: 11444-11449).
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies used to develop new lines of plants that exhibit tolerance to ABS are relatively inefficient since they are tedious, time consuming and of unpredictable outcome. Furthermore, limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to ABS tolerance are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways.
Genetic engineering efforts, aimed at conferring abiotic stress tolerance to transgenic crops, have been described in the art. Studies by Apse and Blumwald (Curr Opin Biotechnol. 13:146-150, 2002), Quesada et al. (Plant Physiol. 130:951-963, 2002), Holmstrom et al. (Nature 379: 683-684, 1996), Xu et al. (Plant Physiol 110: 249-257, 1996), Pilon-Smits and Ebskamp (Plant Physiol 107: 125-130, 1995) and Tarczynski et al. (Science 259: 508-510, 1993) have all attempted at generating stress tolerant plants.
In addition, several U.S. patents and patent applications also describe polynucleotides associated with stress tolerance and their use in generating stress tolerant plants. U.S. Pat. Nos. 5,296,462 and 5,356,816 describe transforming plants with polynucleotides encoding proteins involved in cold adaptation in Arabidopsis thaliana for promoting cold tolerance.
U.S. Pat. No. 6,670,528 describes transforming plants with polynucleotides encoding polypeptides binding to stress responsive elements for promoting tolerance to abiotic stress.
U.S. Pat. No. 6,720,477 describes transforming plants with a polynucleotide encoding a signal transduction stress-related protein, capable of increasing tolerance of the transformed plants to abiotic stress.
U.S. application Ser. Nos. 09/938,842 and 10/342,224 describe abiotic stress-related genes and their use to confer upon plants tolerance to abiotic stress.
U.S. application Ser. No. 10/231,035 describes overexpressing a molybdenum cofactor sulfurase in plants for increasing tolerance to abiotic stress.
WO2004/104162 to Evogene Ltd. teaches polynucleotide sequences and methods of utilizing same for increasing the tolerance of a plant to abiotic stresses and/or increasing the biomass of a plant.
WO2007/020638 to Evogene Ltd. teaches polynucleotide sequences and methods of utilizing same for increasing the tolerance of a plant to abiotic stresses and/or increasing the biomass, vigor and/or yield of a plant.
WO2007/049275 to Evogene Ltd. teaches isolated polypeptides, polynucleotides encoding same for increasing tolerance of a plant to abiotic stress, and/or for increasing biomass, vigor and/or yield of a plant.
Additional background art includes U.S. Patent Appl. Nos. 20060183137A1 A1 and 20030056249A1.